This invention is related to a gas mixture and method for metal inert gas (MIG) brazing.
Presently, the performance of the existing shielding gases for MIG brazing of galvanized steels requires optimizing for each narrow coating layer thickness range. Whenever this thickness exceeds certain limits, poor arc stability occurs in the brazing process. This poor arc stability produces detrimental effects in the bead geometry and finishing, as well as produces a remarkable reduction in the corrosion resistance of the heat affected zone. Most often, the arc""s instability originates from excessive amounts of zinc oxide formation in the arc""s column.
At present, commercial gas mixtures rely upon oxygen additions to argon to stabilize the arc and thereby solve the problems of spatter generation. Unfortunately, oxygen detrimentally contributes to further zinc oxidation. Consequently, the oxygen damages the base metal coating layer, which affects the corrosion resistance of the joint. As discussed above, another limitation of these argon-oxygen mixtures is that the arc stability produced by the oxygen additions is restricted to a narrow coating thickness range.
Publications relating to shielding gas mixtures for MIG brazing include: Japan Patent No. JP9248668A2; WO Patent No. WO024545A1; H. Hackl, xe2x80x9cMIG brazing of Galvanized Light-Gauge Sheetxe2x80x9d, Fronius, Austria; G. Davies, xe2x80x9cPre-coated Steel in the Automotive Industryxe2x80x9d, MBM, May/1993; U. Dilthey et all, xe2x80x9cGMA-Brazing of Galvanized and Alloyed Steels. An Alternative Joining Technology in Vehicle Constructionxe2x80x9d, IIW, doc XII. As far as known, none of these shielding gases produce excellent arc stability independent of a coating""s thickness; reduce the damage to this coating layer; or maintain the corrosion resistance of the welded parts.
The shielding gas mixture is useful for MIG brazing galvanized steel. The gas mixture consists essentially of, by volume percent, about 0.5 to 4 carbon dioxide, about 0.2 to 1 hydrogen, and the balance is argon and incidental impurities. A MIG process uses this shielding gas to braze galvanized steels. First, an electric arc forms between a consumable copper-base alloy wire electrode and a workpiece. Then the shielding gas mixture shields the arc for MIG brazing the workpiece with the copper-base alloy wire. The shielding gas limits zinc oxidation of the galvanized steel and stabilizes the arc.
An argon gas mixture containing carbon dioxide and hydrogen unexpectedly improves MIG brazing. The argon-base gas mixture achieves its performance through the carbon dioxide and hydrogen additions. During MIG brazing, dissociation and recombination of the carbon dioxide and hydrogen molecules in the arc zone generate heat and improve both bead finishing and geometry. In addition, hydrogen provides a reducing environment that decreases zinc oxidation, improves arc stability and maintains corrosion resistance. Furthermore, the improved arc stability generates less spatter, improves bead geometry and appearance, enhances mechanical properties and maintains corrosion resistance of the brazed joint for a wide thickness range of the base metal""s coating layer. For purposes of this specification, MIG brazing includes: arc brazing processes having an electric arc established between a consumable copper-base alloy wire electrode and a workpiece; and continuously feeding the copper-base alloy wire to this arc as it moves with respect to a workpiece""s joint.
The shielding gas mixture operates in all welding (brazing) positions, both for manual and automatic brazing in short circuit or pulsed spray transfer modes.
The arc and the molten wire are protected by a shielding gas from the contaminants of the atmosphere. This shielding gas, as provided in the present invention, contains, by volume percent, from about 0.5 to 4 volume percent carbon dioxide. This specification identifies all concentrations by volume percent, unless specifically referenced otherwise. Advantageously, the shielding gas contains about 1 to 3 volume percent carbon dioxide for heating purposes. Most advantageously, the shielding gas contains a nominal concentration of about 2 volume percent carbon dioxide. In addition, the shielding gas contains from about 0.2 to 1 percent hydrogen to prevent zinc oxidation and to stabilize the arc. Advantageously, the stabilizing gas contains about 0.25 to 0.75 volume percent hydrogen; and most advantageously contains a nominal concentration of about 0.5 percent hydrogen. The balance of the stabilizing gas is argon and incidental impurities.
The stabilizing gas is most effective for hot dip galvanized steel and copper-base filler metals. This gas has proved particularly effective for copper-base alloys such as, AWS A5.7 ERCuSixe2x80x94A (having a nominal composition of 94% Cu+0.6% Sn+0.3% Fe+0.8% Mn+3.3% Si+0.6% Zn) and 97% Cu+0.2% Sn+0.2% Fe+0.8% Mn+1.3% Si, by weight percent.